Main reflector and feed system with aperture blocking correction



Aug. 31, 1965 R. RUBIN ETAL 3,204,243

MAIN REFLECTOR AND FEED SYSTEM WITH APER'I'URE BLOCKING CORRECTION Filed May 29, 1961 E (VOLTS) E (VOLTS) 3 Sheets-Sheet l -3DB BEAMWIDTH [.21

APERTURE BLOCKED APERTURE UNBLOCKED st LOSE lst SlDE-LOBE IO -23.| DB

2nd SlDE-LOBE vfi 0.57r 1r L511 21 2.51r 31r 3-51r 41r NORMALIZED SPACE ANGLE, 1:];

Fig. I

-3 DB BEAMWIDTH |.s4 4O -3 DB BEAMWIDTH L54 INVENTORS DOMINIC FERRANTE RICHARD RUBIN AUGUSTINE R. STRATOTI SPACE ANGLE. DEGREES) Fig. 4

Aug. 31, 1965 R. RUBIN ETAL MAIN REFLECTOR AND FEED SYSTEM WITH APERTURE BLOCK 1N6 CORRECTION Filed May 29, 1961 5 Sheets-Sheet 2 Fig. 2

INVENTORS DOM l NIC FERRANTE RICHARD RUBIN AUGUSTINE R. STRATOTI BY/ wad ATTORNEY g 31, 1965 R. RUBIN ETAL 3,204,243

MAIN REFLECTOR AND FEED SYSTEM WITH APERTURE BLOCKING CORRECTION Filed May 29, 1961 3 Sheets-Sheet 3 SCANNER 8.LENSES SCANNER 8 LENSES AXIS 0F PROPAGATION INVENTORS DOMINIC FERRANTE RICHARD RUBIN AUGUSTINE R. STRATOTI ATTORNEY United States Patent O enemas MAEN REFLECTQR AND FEED SYSTEM WITH AYERTURE BLO'CKHQG CGRRECHUN Richard Rubin, Natick, Dominic Ferraute, Wayland, and Augustine R. Stratoti, Westwood, Mash, assignors to Sylvania Electric Products lino, a corporation of Delaware Filed May 29, lfidl, der. No. 113,447 Mathis. (Ci. 343-778) This invention relates to directive microwave antennas and particularly to directive antennas of the type including a reflector and a feed system.

A common form of directive antenna system, having utility, for example, in a radar system, consists of a cylindrical parabolic reflector, and a feed element positioned generally at the focal line of the reflector. During transmission, the feed element is excited by a suitable transmitter and directs energy onto the reflecting surface of the reflector. The reflector focuses the energy to form a beam of radiation which is propagated into space. During reception of energ such as a returned target signal, the reflector collects the signal energy and focuses it onto the feed element. The transmitter is decoupled from the feed element during reception, and a receiver substituted to detect and display the signal arriving at the feed element. Many forms of feed elements are known to the art, such as horns, dipoles, dielectric guides, etc. The reflector, also, may take a variety of shapes, but generally is either a paraboloid, to produce a narrow beam, or a truncated paraboloid to produce a beam which is narrow in the horizontal plane and broad in the vertical plane.

The manner in which energy is fed to and from the feed element depends to some extent on the type of reflector used, but as a general proposition the feed element is located at the focal point or line of the reflector to achieve optimum collimation of the energy and maximum transmission and reception efiiciency. On the other hand, this positioning of the feed element, which is usually formed of conductive material, within the aperture of the reflector constitutes an obstruction to energy from the reflector and gives rise to what is known in the art as aperture blocking. The degradation of antenna performance due to aperture blocking and specific eflfects thereof, are illustrated in FIG. 1 of the drawings which compares the sum far-field radiation pattern of an antenna with 8.8% of its aperture blocked with that of an unblocked antenna. The plots of the two radiation patterns of FIG. 1 were derived by solving, for blocked and unblocked antennas having a truncated paraboloidal reflector, the Fourier integral equation:

where E is the far-field radiation intensity, a is the height of the truncated paraboloidal reflector, f(x) is the electric field distribution across the aperture of the reflector, B is the propagation constant equal to 21/ 7\, and 0 is the space angle measured from the axis of the reflector. It will be seen that the blocked antenna has lower main beam gain and higher side lobes than the unblocked antenna. That is, a significant portion of the radiation of the blocked antenna is diverted from the main beam and wasted in the side lobe field, thus reducing the efficiency of the antenna system.

It would appear at first impression that aperture blocking could be reduced by decreasing the cross-sectional area or" the feed element to thereby reduce the size of the obstruction. It is fundamental, however, that the surface area of the reflector illuminated by the feed varies inversely as the size of the feed element; this is particularly true in the case of a horn or parallel-plate line source 3,264,243 Patented Aug. 31, 1965 "ice feed. Thus, there is a limit below which the size of the feed element can be reduced before part of the illuminating energy will spill over the perimeter of the reflector and be wasted. Another approach for reducing aperture blocking would be to employ sub-reflectors with polarization twisting contours and wire gratings, such as those used in Cassegrainian reflectors, which make the feed lines appear transparent to a particular polarization field to thus reduce the apparent size of the obstruction. Devices of this kind are quite frequency sensitive, however, besides being bulky and expensive, and therefore are not a satisfactory solution.

Accordingly, it is a primary object of the present invention to reduce the deleterious effects of aperture blocking.

Another object of the invention is to provide a directive antenna system of the type including a reflector and feed element in which the aperture blocking ettect of the feed element is reduced.

Another general object of the invention is to improve the gain and reduce the side lobes of antenna systems of the type including a reflector and a center feed.

Still another object of the invention is to provide a center-fed reflector antenna having the foregoing features which is relatively simple and inexpensive to construct.

The invention is particularly applicable to center-fed reflector antennas and will be described in a system embodying a reflector of truncated paraboloidal shape illuminated by a central line-source feed element. This known type of feed consists of two separate parallel-plate transmission lines dimensioned to propagate in the TEM mode which project from the back through the center of the reflector and are'curved back on themselves at the focal line to illuminate the reflector. This structure positioned on the axis of the reflector produces the block ing effect to which the PIcSEIl'E invention is addressed.

Briefly, the invention contemplates the provision or" a separate radiator positioned in that portion of the aperture of the antenna which is normally blocked for radiating signal energy directly into space to fill in the blocked portion of the main beam. The signal radiated by this auxiliary radiator, which may be in the form of a horn, is in phase with the signal radiated from the reflector when it reaches the aperture of the antenna so as to add as they are launched into space. Thus, the signal from the fill-in radiator reinforces the main beam of the antenna system, and concomitantly materially suppresses undesirable side lobes.

ln an embodiment of the invention in an antenna system employing two parallel-plate lines as the feed elernent, the fill-in horn is mounted on the axis of the feed lines and is excited by tapping into one or both of the main feed lines. By proper adjustment of the electrical length of the transmission lines to the horn, which may be accomplished by adjusting their physical length or by the appropriate use of dielectric loading, the fields radiated from the reflector and horn are in phase. The Width of the horn, as measured along the focal line of the reflector may be of approximately the same width as the main feed element, and the invention therefore can be used with equal advantage in scanning and non-scanning antenna systems. A single fill-in horn may be used, or, should system requirements dictate, the horn may be divided into two separate channels. For example, a divided horn can be used to advantage in a monopulse radar system because of the slight improvement of the difference pattern sensitivity; conversely, dividing the horn does not significantly aflect the sum radiation pattern of the antenna.

Other objects, features, and advantages of the invention, and a better understanding of its construction and operation, will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a plot of calculated sum far-field radiation patterns of blocked and unblocked antennas, to which reference has been made earlier;

FIG. 2 is a perspective view of a directive antenna system, employing parallel-plate feed lines, in which the present invention is incorporated;

FIG. 3 is an elevation cross-section view of FIG. 2;

FIG. 4 is a plot of radiation patterns illustrating the improvement achieved with the fill-in horn of the invention over a blocked antenna; and

FIG. 5 is an elevation cross-section of an antenna system illustrating an alternate method of energizing the fill-in horn.

Referring now to FIGS. 2 and 3, one form of the invention will be described as embodied in an antenna system having a cylindrical parabolic reflector energized by a pair of parallel-plate feed lines. This known type of antenna feed consists essentially of a pair of parallelplate waveguides 12 and 14- having a common or contiguous broad wall 16, which extends through the reflector 1i) along the directive axis of the parabolic cylinder. These waveguides are bent back on themselves along lines somewhat beyond the focal line of the reflector whereby their respective open ends 12' and 14 are directed back toward the reflector. The open ends of the two guides are disposed in a common vertical plane which includes the focal line of the reflector. Normally, the narrow dimension of the two waveguides is uniform throughout their length, the illustrated construction constituting a departure from the normal case to accommodate the present invention. Suflice it to say, this type of antenna feed provides two parallel line sources of energy lying substantially in the vertical plane which includes the focal line of the reflector. In certain types of scanning antennas, the width of the feed as measured in the direction parallel to the focal line of the reflector may be approximately the same length as the cylindrical parabolic reflector, or, in non-scanning systems, the width may be considerably smaller than the length of the reflector. In any event, the feed element constitutes an obstruction to energy reflected from reflector It the area of the obstruction being determined by the width, w, and the height, h, of the feed structure.

In accordance with the present invention, a horn radiator is incorporated into the parallel plate feed structure to radiate signal energy directly into space to reinforce that portion of the main beam obstructed by the feed lines 12 and 14. In the embodiment of FIGS. 2 and 3, waveguides 12 and 14 are modified to define a horn while also serving to propagate energy to illuminate the reflector. More specifically, the common or contiguous wall 16 of the main waveguides 12 and 14 are tapered toward the front of the feed assembly and, as will be seen, divides the horn into two parts. Starting at a point rearwardly of the reflector, main waveguides 12 and 14 are each divided by conductive walls 18 and 20, respectively, to form two pairs of contiguous waveguide structures. The upper broad wall of waveguide 12 and the dividing septum 18 define an air dielectric waveguide for propagating energy to be directed toward the reflector, and similarly the lower broad wall of waveguide 14 and its septum 20 define the other waveguide feed. Similarly, septum 18 and common wall 16 define a wave propagating structure, as does common wall 16 and septum 20. The latter two waveguides constitute the two halves of the fill-in horn. It should be noted that the dividing walls 18 and 2t) divide the power propagated in the main waveguides between corresponding feed lines and the two halves of the horn.

For the fill-in horn to achieve its purpose, the fields radiated by the reflector and the fill-in horn must produce a substantially uniform phase front across the aperture of the antenna. To achieve this, it is necessary that the signals propagated in main waveguides 12 and 14 undergo the same phase delay before arriving at the aperture of the antenna whether propagated through the feed lines and reflected from the reflector or propagated directly through the horn. In other words, the electrical length l of the horn as indicated in FIG. 3 must be equal to the sum of the electrical length l of a reflector feed line plus twice the electrical distance 1" between the reflector and the aperture of the antenna. Since the horn in the disclosed embodiment is of substantially the same physical length as the distance between the reflector and the antenna aperture, the component of the input signal which is reflected must travel approximately three times as far as the component propagated by the horn. To achieve the necessary phase delay in the fill-in horn, the two halves of the horn are filled with slabs of dielectric material 20 and 22 having a dielectric constant to introduce the requisite phase delay. Since the phase shift experienced by a signal in a dielectric material varies inversely as the square root of the dielectric constant, the dielectric slabs 2t) and 22 have a dielectric constant of approximately 9 to insure that the reflected and horn components of the input signal arrive in phase at the aperture of the antenna. The curvature of the reflector feed lines at their extremities produce a flare at the aperture of the horn whereby the aperture of the horn is substantially equal to the size of the obstruction caused by the feed element. However, should it be found that the radiation pattern of the antenna system can be optimized by decreasing the size of the aperture of the fill-in horn, this can be readily accomplished by shaping the outer ends of the dielectric slabs.

As was mentioned earlier, the structure illustrated in FIGS. 2 and 3 is intended for scanning operation, to scan a narrow beam of radiation back and forth across the linear dimension of the aperture of the system. This is accomplished by coupling signal energy to the main feed lines 12 and 14 from a pair of separate scanning devices 24 and 26, each of which might also include lenses for shaping the radiation pattern emanating from the scanner. These scanners, which form no part of the present invention, periodically sweep a beam of microwave energy back and forth across the wide dimension of their respective main waveguide. The signal energy is propagated in the TEM mode in the waveguides, which are dimensioned to prevent spreading throughout the width of the guide as the energy is propagated from the scanner to the open ends 12' and 14' of the guides. That is, at any instant in a sweeping cycle only a segment of the total length of the feed, designated .9, is energized whereby the beam formed by the reflector at the same instant is of comparable width. The signals in the main waveguides are divided by the septa 18 and 20 causing part of the energy in each main guide to be propagated to the reflector feed and part to be launched into a corresponding part of the horn. Accordingly, at the indicated instant of time, the horn is also energized over only a portion of its width w, designated s. Consequently, as the waves reflected from the reflector arrive at the aperture of the antenna they add to the field produced by the horn to form a beam having an effective width approximately equal to s. As the input signal is swept across waveguides 12 and 14, the energized segment of the reflector feeds and of the horn correspondingly move with the result that the reinforced main beam is scanned across the antenna system.

Although the invention has been described as it would apply to a scanning antenna, it will be appreciated that the advantages of the fill-in horn can be equally realized in an antenna of the non-scanning type. In the latter case, the feed structure may be somewhat narrower in the direction parallel to the focal line of the reflector and the entire width uniformly energized at all times during operation.

The advantages of the fill-in horn will be apparent from the plot in FIG. 4. Curve D is the unnormalized form of the blocked antenna pattern previously shown as curve B in FIG. I, and the dotted curve'E represents the radiation pattern produced by the horn alone. The normalized antenna pattern of the blocked antenna (curve B of FIG. I) was converted to the unnormalized form of FIG. 4 by solution of the equation,

where 0 is the space angle, at is the normalized space angle used in FIG. I. A isthe operating wavelength, and a is the height of the reflector 10. Inasmuch as the signal propagated from an antenna undergoes a 180 phase reversal at points of zero field intensity, the main beam and first side lobe of curve D are of opposite phase as indicatcd by the plus and minus signs. The second side lobe of curve D, beyond the limits of FIG. 4 and therefore not shown, is in phase with the main beam but has an amplitude less than that of the first side lobe. The field strength of the signal from the horn, curve B, gradually tapers with increasing space angle. If the plots were continued beyond the space angle limits of FIG. 4 (to space angles corresponding to points far beyond the aperture of the horn) the field strength would show a more rapid taper. Calculations have shown that the plot E exhibits approximately a 15 db loss from its maximum at space angles corresponding to the location of the second side lobe of curve D.

Since the resultant beam is due to the combination of the fields produced by the blocked reflector and by the fill-in horn, the resulting radiation pattern, curve C, was determined by algebraically adding plot E to curve D. Considering for the present only the most significant portions of the two patterns, namely, within the space angle limits of FIG. 4, the horn signal, being of the same phase, increases the main beam segment D; to the level represented at C,. On the other hand, because the signal in the first side lobe is of opposite phase relative to the horn signal, the irregular first side lobe is reduced to the levels indicated at C, and C;. To summnrize, the presence of the fill-in horn increases the field strength in the main beam and decreases the major side lobe, thus causing a substantial improvement over the pattern of a blocked antenna. The horn signal is in phase with the second side lobe also, which is therefore also reinforced, but since the amplitude of the horn signal at phase angles where the second side lobe appears is extremely small, the resulting increase in that minor lobe is virtually negligible.

It should be noted in connection with curve A of FIG. 1 that when the second lobe is close to the main beam the invention cannot be used to the same advantage. In other words, the fill-in horn gives the greatest improvement in antennas having a blocked aperture and accompanying relatively wide first side lobe. In the case of an ideal antenna with an unblocked aperture, the horn signal, while decreasing the first side lobe, would unavoidably increase the second lobe to a level greater than the original level of the first lobe and the net result would be a worsening of the antenna pattern.

FIG. 4 shows the resultant radiation pattern for one value of horn signal intensity, but it will be appreciated that the radiation pattern, both in-the main beam and side lobe regions, can be optimized by varying the intensity of the horn signal. This may be done by changing the cross-sectional area of the horn feed to receive a larger or smaller portion of the signal propagated in the main feed lines, or by changing the size of the aperture of the born to increase or decrease its gain. To achieve the radiation pattern 0 of FIG. 4, 12.3 db of the signal in the main feed lines is coupled to the horn. If the horn feeds were made larger, or the aperture of the horn made larger, the level of curve B would be increased. The

far-field radiation pattern of the reflector and fill-in horn combination can be calculated by calculating the individual far-field pattern of the blocked reflector and of the fill-in horn by means of the Fourier integral described earlier in this discussion and algebraically adding the resultant far-field patterns. It is cautioned, however, that although increasing the horn signal intensity has the tendency of diminishing the odd side lobes, it also tends to increase the level of the even side lobes which are in phase with the horn signal. Accordingly, the signal level from the horn must be selected to reach a satisfactory compromise where the odd side lobes are reasonably compensated and the even side lobes at large space angles do not become bothersome.

The divided horn of FIGS. 2' and 3 is preferred in a scanning antenna because of the ease of fabricating the conductive surfaces of the horn into the main feed lines, but through the utilization'of other wave transmission structures, the dividing septum can be omitted without significantly altering the operation of the horn. It has been observed that dividing the horn has little or no effect on the sum radiation pattern of the antenna system, but that the difference radiation pattern is slightly increased in overall field intensity, both in the main beam and the side lobes. Thus, a divided horn may be used to advantage in monopulse systems to slightly improve the difi'erence pattern sensitivity, providing the accompanying increase in side-lobe level is tolerable.

While the dielectric loaded horn of FIGS. 2 and 3 has the particular advantage of not appreciably increasing the size of the feed structure, the invention is not limited to this arrangement. That is, other physical structures may be used to achieve the appropriate phase relationship between the reflected signal and the signal from the horn. For example, as shown in FIG. 5, the main feed waveguides 12 and 14 may be divided at 18' and 20', respectively, to split the energy propagated in these waveguides. A portion of the energy in each is propa- V gated through the open ends 12' and 14' of the turnedback feed lines to illuminate the reflector l0, and the remaining portion of the energy in each is propagated through feed lines 30 and 32 to the horn 34, which is disposed on the axis of propagation of the reflector. The horn is divided into two sections by a conductive septum 36 which may be a common wall for the waveguides 30 and 32 in the horn portion of their length, or it may represent contiguous side walls'of guides 30 and 32. The flare of the horn may be varied to control its gain and hence its contribution to the resultant radiation pattern from the antenna in a manner similar to that discussed in connection'with FIGS. 2 and 3. Waveguides 30 and 32 are filled with air and, accordingly, in order for the horn signal to be in phase with the reflected signal must be of a length approximately equal to the distance from the reflector feeds to the reflector and back to the plane of the aperture of born 34, appropriate account being taken for the somewhat slower propagation in a conductively bounded waveguide than in free space. It should be pointed out that the illustration of FIG. 5 is somewhat diagrammatic in that the dimensions of the feed clement have been exaggerated for clarity and hence disproportionately large relative to the reflector. Although differing substantially in physical structure from the dielectric loaded hom of FIGS. 2 and 3, the antenna of FIG. 5 has comparable performance characteristics. Another means for adjusting the line lengths is to employ variable phase shifters in the lines feeding the fill-in horn.

From the foregoing, it is seen that applicants have providcd a directive antenna system in which the deleterious eflects of aperture blocking normally present in centerfed reflector antenna systems are overcome by the use of a fill-in horn in the region of the feed element. By proper design of the horn and its feed lines, the signal energy propagated from the horn is in phase with the signal wave front propagated from the reflector and fills in" a cross-sectional area substantially equal to that of the obstruction. Thus, the signal propagated from the.

horn adds directly to the energy from the reflector, eliminating the efl'ects of the obstruction. By adiusting the division of power to the horn, or the size of the aperture of the born, the intensity of the horn signal may be adjusted to obtain the desired resultant radiation pattern. The horn is fabricated from parallel-plate waveguide which is useful over relatively broad ranges of frequency.

Although there is shown and described what applicants now consider to be preferred embodiments of the invention, modifications will occur to those skilled in they art, including other physical structures for insuring that the horn and reflector fields are in phase at the aperture of the antenna, and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A directive antenna comprising, a curved reflector having an axis of propagation, a pair of feed lines disposed about said axis of propagation and arranged to direct a portion of the signal from a transmitting source toward said reflector, a horn antenna interposed between said feed lines for radiating energy directly into the media of transmission, and means coupled between said feed lines and said horn antenna for coupling a portion of the signal from said source to said horn antenna and including means for shifting the phase of the signal propagated therein whereby the horn and reflector component signals form a wavefront of substantially uniform phase for propagation into the media of transmission.

2. A directive antenna comprising a curved reflector having an axis of propagation, a source of microwave energy, a first pair of feed lines disposed in front of said reflector along the axis of propagation for directing a portion of the signal from said source of energy toward said reflector, a horn antenna interposed between said feed lines for directly radiating a portion of the signal from said source into the transmitting media, and a sec- 0nd pair of feed lines coupled between respective ones of said first pair of feed lines and said horn and having an electrical length to cause the horn signal to be substantially in phase with the signals reflected from said reiieCtOt as said signals are propagated into the transmitting media.

'8 3. A directive antenna comprising, a cylindrical paraboloidal reflector havinga focal line and an axis of pr'opagatioma pair of parallel-plate feed lines disposed along said axis of propagation and having open ends substantially at said focal line directed toward said reflector fordirecting energy toward said reflector, a horn radiator interposed between said feed lines for radiating energy directly into free space, means for coupling energy from said feed lines to said horn radiator, and dielectric material in said horn radiator having a dielectric constant of a value to cause the signal radiated from said horn to be in-phase with the energy reflected from said reflector at the aperture of said reflector.

4. A directive antenna comprising, a cylindrical paraboloidal reflector having a focal line and an axis of propagation, a source of microwave energy, first and second contiguous hollow waveguides coupled to said source and projecting'throughsaid reflector along said axis of the. propagation, dividing walls within said first and second waveguides dividing each into a pair of channels, a corresponding one of the channels in each guide being turned back toward said reflector substantially at saidfocal line to direct a portion of the energy from said source toward said reflector, the other two channels formed by said divided waveguides constituting a diyidcd'horn having its aperture disposed substantially at said focal line, said other two channels containing dielectric material having a dielectric constant to cause the signal radiated directly into the transmission media by said horn to be in phase with the signal reflected from said reflector.

' References Cited by the Examiner UNITED STATES PATENTS 2,018,273 10/35 Lux. 343-838 2,415,089 2/47 Feldmsn 343-776 2,698,901 1/55 Wilkes 343-781 2,736,895 2/56 Cochrane 343-756 2,824,305 2/58 Ohlemacher 343-776 3,032,761 5/62 Msnthey 343-758 3,071,770 1/63 Wilkes 343-781 HERMAN KARL SAALBACH, Primary Examiner. GEORGE N. WESTBY, ELI LIEBERMAN, Examiners. 

1. A DIRECTIVE ANTENNA COMPRISING, A CURVED REFLECTOR HAVING AN AXIS OF PROPAGATION, A PAIR OF FEED LINES DISPOSED ABOUT SAID AXIS OF PROPAGATION AND ARRANGED TO DIRECT A PORTION OF THE SIGNAL FROM A TRANSMITTING SOURCE TOWARD SAID REFLECTOR, A HORN ANTENNA INTERPOSED BETWEEN SAID FEED LINES FOR RADIATING ENERGY DIRECTLY INTO THE MEDIA OF TRANSMISSION, AND MEANS COUPLED BETWEEN SAID FEED LINES AND SAID HORN ANTENNA FOR COUPLING A PORTION OF THE SIGNAL FROM SAID SOURCE TO SAID HORN ANTENNA AND INCLUDING MEANS FOR SHIFTING THE PHASE OF THE SIGNAL PROPA- 